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SD and AR are employees of Sanofi Pasteur. The remaining authors declare no competing interests. The National Institute of Public Health in Prague is partner of the Global Influenza Hospital Surveillance Network (GIHSN) and received support for this study from the Foundation for Influenza Epidemiology, partly funded by Sanofi Pasteur, and under the auspices of Fondation de France. JK is the national project coordinator of the GIHSN for the Czech Republic. Sanofi Pasteur also funded the medical writing support for the preparation and submission of the manuscript to the journal.

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